[ http://issues.apache.org/jira/browse/MODPYTHON-114?page=all ]
Graham Dumpleton reassigned MODPYTHON-114:
------------------------------------------
Assign To: Graham Dumpleton
> Problems with PythonPath directive.
> -----------------------------------
>
> Key: MODPYTHON-114
> URL: http://issues.apache.org/jira/browse/MODPYTHON-114
> Project: mod_python
> Type: Bug
> Components: core
> Versions: 3.1.4, 3.2.7
> Reporter: Graham Dumpleton
> Assignee: Graham Dumpleton
>
> The "PythonPath" setting can be used to define the value of the Python
> "sys.path" variable. It is this variable which defines the list of
> directories that Python will search in when looking for a module to be
> imported.
> Although the actual reassignment of "sys.path" by mod_python does not in
> itself present a problem due to assignment in Python being thread safe by
> definition, the context in which the assignment occurs is not thread safe and
> a race condition exists.
> This exists as the top level mod_python dispatcher will consult the existing
> value of "sys.path" and the last value for the "PythonPath" setting
> encountered before then making a decision to modify "sys.path". If multiple
> requests are being serviced in distinct threads within the context of the
> same interpreter instance, and each at the same time decide they want to
> modify the value of "sys.path", only one might ultimately succeed in setting
> it to the value it wants and any modification required by the other may be
> lost.
> In the worst case scenario, this can result in the importation of any
> subsequent modules within that request failing due to a required directory
> not being present in "sys.path". It is possible that this situation may
> resolve itself and go away on a subsequent request, but due to how mod_python
> caches the last value of "PythonPath" in a global variable this will be
> dependent on what other requests arrive.
> At the least, for mod_python to resolve the problem itself would require a
> request to arrive in the interim which targeted the URL which was not the
> last to cache its raw setting for "PythonPath". This only works though due to
> a further issue whereby alternate requests against URLs with different
> "PythonPath" settings will cause "sys.path" to be extended everytime if the
> "PythonPath" setting references "sys.path". This results in "sys.path"
> continually growing over time due to directories being added multiple times.
> The problematic code in apache.py is:
> if config.has_key("PythonPath"):
> # we want to do as little evaling as possible,
> # so we remember the path in un-evaled form and
> # compare it
> global _path
> pathstring = config["PythonPath"]
> if pathstring != _path:
> _path = pathstring
> newpath = eval(pathstring)
> if sys.path != newpath:
> sys.path[:] = newpath
> To fix the problems, the processing of PythonPath directive should be
> protected by a thread mutex lock and a dictionary should be used to store all
> seen values of PythonPath rather than a single variable. If a value for
> PythonPath has ever been seen, and not just from the last change, then no
> update would be necessary.
> if config.has_key("PythonPath"):
> try:
> _path_cache_lock.acquire()
> pathstring = config["PythonPath"]
> if not _path_cache.has_key(pathstring):
> newpath = eval(pathstring)
> _path_cache[pathstring] = None
> sys.path[:] = newpath
> finally:
> _path_cache_lock.release()
> There shouldn't really be a need to check whether the new path is different
> to the current value of sys.path as that scenario shouldn't occur at that
> point.
> The two parts of this problem were previously catalogued as ISSUE 15 and
> ISSUE 16 in my list of problems with the module importing system. The list
> can be found at:
> http://www.dscpl.com.au/articles/modpython-003.html
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